Osteogenic implants with combined implant materials, and materials and methods for same

ABSTRACT

Described are osteogenic implants that include a first implant material covered at least in part by a second implant material carrying an osteogenic protein such as a bone morphogenic protein. The first implant material can comprise a mineral and provide an inner scaffolding portion for supporting bone ingrowth, and the second implant material can comprise a collagen or other sponge carrier covering the first implant material and having a liquid osteogenic protein formulation imbibed therein. Related implant materials and methods of preparation and use constitute additional aspects of the invention.

REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent application Ser. No. 11/118,124 filed Apr. 29, 2005, now abandoned, which claims the benefit of U.S. Patent Application Ser. No. 60/611,527 filed Sep. 20, 2004, now abandoned, each of which is hereby incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

The present invention resides generally in the field of implants for promoting bone growth, and in one particular aspect the invention relates to implants for promoting bone growth that contain an osteogenic protein combined with a porous matrix material.

As further background, a wide variety of therapeutic regimens are undertaken to induce the growth of bone of a patient into a desired region. Examples of such therapeutic regimens exist in the field of spinal surgery, including a variety of spinal fusion procedures. Illustratively, in posterolateral fusion procedures, bone growth is induced to fuse transverse processes of adjacent vertebrae, typically in the lumbar spine. In the predominant historic and current practice, bone of the patient harvested from the iliac crest is implanted between transverse processes of the patient to facilitate the growth of a bone mass sufficient to achieve arthrodesis. However, increased costs and risks are associated with the harvest of the patient's bone, and in some patients there may be insufficient quality iliac crest bone for the procedure. Consequently, more recent efforts in academics and industry have explored the development of procedures that minimize or eliminate the need to harvest patient bone.

In certain areas of study, implants including osteogenic proteins have been used instead of or as a supplement to autogeneous bone. The use of such osteogenic proteins is itself accompanied by a variety of challenges. The active protein materials are commonly complicated to obtain or produce, costly, and highly regulated. As well, challenges are presented in determining the optimal and most effective use of the osteogenic proteins to generate relevant masses of bone for fusion or other purposes.

In light of this background, there remain needs for improved and/or alternative osteogenic implant materials as well as related materials and methods for their preparation and use. The present invention is addressed to these needs.

SUMMARY OF THE INVENTION

In certain aspects, the present invention features the discovery of an osteogenic implant configuration that effectively utilizes osteogenic protein to induce bone growth through a desired volume occupied by the implant. Accordingly, in one embodiment, the present invention provides an osteogenic implant for promoting bone growth between first and second bone surfaces. The implant includes a first resorbable implant material defining an implant body configured for receipt between the first and second bone surfaces. The first resorbable implant material includes a porous collagenous matrix containing mineral particles. The osteogenic implant also includes a second resorbable implant material covering at least a portion of the outer surface of the implant body, wherein the second material is positioned to contact the first and second bone surfaces. The osteogenic implant further includes an osteogenic protein carried by the second resorbable implant material. In certain embodiments, the osteogenic implant is configured for receipt between adjacent upper and lower transverse processes in the spine of a mammal, including a human, and/or the osteogenic protein is a bone morphogenic protein (BMP) such as BMP-2, BMP-4, BMP-6, or BMP-7. As well, the mineral particles can comprise bone, a synthetic ceramic material, or combination thereof.

Another embodiment of the invention provides a medical implant for promoting bone growth, the medical implant having an outer osteogenic implant material, the outer osteogenic implant material including a wetted, porous bioresorbable sponge matrix having sorbed therein an aqueous medium including an osteogenic protein. The implant also includes an inner scaffolding implant material including a mineral component such as an osteoconductive synthetic ceramic, the inner scaffolding implant material configured to occupy a three-dimensional volume for bone ingrowth initiated by the outer osteogenic implant material.

In another embodiment, the invention provides an implant configured to promote spinal fusion between first and second transverse processes in a patient. The implant includes a first resorbable implant material defining an implant body configured for receipt between the first and second transverse processes, wherein the first resorbable implant material includes a porous collagen-containing matrix incorporating mineral particles. A collagen sponge carrier covers at least a portion of the outer surface of the implant body and is positioned to contact the first and second transverse processes. A bone morphogenic protein is carried by the collagen sponge carrier.

In another embodiment, the invention provides an implant for promoting bone growth between first and second bone surfaces. The implant includes a first implant material containing collagen and mineral and configured to occupy a volume for bone growth. The implant further includes a second implant material covering at least a portion of the first implant material, wherein the second implant material carries an osteogenic protein.

In another aspect, the invention provides an implant suitable for carrying an osteogenic protein. The implant includes a first resorbable implant material defining an implant body and configured for receipt between first and second bone surfaces. The first resorbable implant material includes a mineral component, and can be a porous resorbable matrix incorporating mineral particles. A second resorbable implant material is provided a covering at least a portion of the outer surface of the implant body and is positioned to contact the first and second bone surfaces. In certain embodiments, the implant can be configured for use in a spinal fusion procedure in a mammal such as a human, including a posterolateral spinal fusion procedure. The first resorbable implant material can for example include a porous collagen-containing matrix incorporating mineral particles, and/or the second resorbable implant material can include a collagen sponge carrier.

Still another embodiment of invention provides a medical kit for promoting bone growth between first and second bone surfaces of a patient. The medical kit includes a first resorbable implant material comprising a mineral component such as a porous resorbable matrix having particulate mineral embedded therein. The kit further includes a second resorbable implant material configured to cover at least a portion of a surface of the first resorbable implant material, and an osteogenic protein. The kit can include other components such as one or more syringes, surgical tools, and/or surgical implants.

In another embodiment, the invention provides a method for preparing a medical implant for inducing bone growth in a patient at an implant site. The method includes the steps of providing (i) a dry, porous bioresorbable sponge matrix; (ii) an osteoconductive scaffolding material comprising a mineral component; and (iii) an aqueous formulation including an osteogenic protein. The dry, porous bioresorbable sponge matrix is wetted with the aqueous formulation so as to form a wetted bioresorbable sponge matrix having the aqueous formulation sorbed therein, and at least a portion of the osteoconductive scaffolding material is covered with the wetted bioresorbable sponge matrix.

The invention provides in another embodiment a method for inducing spinal fusion between first and second bone surfaces in a patient. The method includes providing (i) a dry, porous bioresorbable sponge matrix; (ii) an osteoconductive scaffolding material comprising a mineral component; and (iii) an aqueous formulation including an osteogenic protein. The dry, porous bioresorbable sponge matrix is wetted with an aqueous formulation so as to form a wetted bioresorbable sponge matrix having the aqueous formulation sorbed therein. The wetted bioresorbable sponge matrix is manipulated to cover at least a portion of the osteoconductive scaffolding material and form a combined implant construct. The combined implant construct includes first and second portions of the wetted porous sponge matrix positioned to contact the first and second bone surfaces, respectively, the combined implant construct further having the osteoconductive scaffolding material positioned between the first and second portions of the wetted bioresorbable sponge matrix. The method further includes implanting the combined implant construct between the first and second bone surfaces in the patient with the first and second portions of the wetted porous sponge matrix contacting the first and second bone surfaces and the osteoconductive scaffolding material occupying a volume for bone growth to create a fusion mass between the first and second bone surfaces.

In still further embodiments, the present invention provides further methods of preparing and using implants of the invention as described hereinbelow.

Additional embodiments as well as features and advantages of the invention will be apparent from the descriptions herein.

DESCRIPTION OF THE FIGURES

FIG. 1 provides a perspective view of first and second resorbable implant materials used to prepare an implant of the invention.

FIG. 2 provides a cross-sectional view of an implant of the invention including first and second resorbable implant materials.

FIG. 3 provides a view of an implant of the invention configured for posterolateral fusion received between first and second transverse processes of a human patient.

FIG. 4 provides a perspective partial cutaway view of another combined implant construct of the invention.

DETAILED DESCRIPTION

For the purposes of promoting an understanding of the principles of the invention, reference will now be made to certain embodiments thereof and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, and alterations and modifications in the illustrated implants, and further applications of the principles of the invention as illustrated herein are contemplated as would normally occur to one skilled in the art to which the invention relates.

As disclosed above, one aspect of the present invention provides osteogenic implants that include a first implant material having a surface covered at least in part by a second implant material, wherein the second implant material incorporates an osteogenic protein such as a bone morphogenic protein (BMP). In other aspects, the invention provides materials and methods for preparing and using osteogenic implants.

Implants of the invention include a first implant material including a natural and/or synthetic mineral component. For example, the mineral component can be provided by a particulate mineral material, including either powder form or larger particulate mineral materials such as granules. In certain embodiments, the particulate mineral component is effective in providing a scaffold for bone ingrowth as the resorbable matrix material is resorbed. The mineral material may for example be bone, especially cortical bone, or a synthetic bioceramic such as a calcium-containing ceramic, for example a biocompatible calcium phosphate ceramic. Illustrative ceramics thus include tricalcium phosphate, hydroxyapatite, and biphasic calcium phosphate. These mineral components may be purchased commercially or obtained or synthesized by methods known in the art. Mineral components of inventive implants can also serve as a source of calcium and/or phosphate ions for bone generation and can be incorporated at levels to regulate the compressibility of the implants.

As noted above, biphasic calcium phosphate can be used to provide the mineral component in the invention. Desirably, such biphasic calcium phosphate will have a tricalcium phosphate:hydroxyapatite weight ratio of about 50:50 to about 95:5, more preferably about 70:30 to about 95:5, even more preferably about 80:20 to about 90:10, and most preferably about 85:15.

The first implant material can include an amount of mineral that will provide a scaffold effective to remain in the patient for a period of time sufficient for the formation of osteoid in the void for which bone growth is desired. Typically, this period of time will be about 8 to about 12 weeks, although longer or shorter periods may also occur in particular situations. The minimum level of mineral that must be present in the composition is also dependent on the activity of the BMP or other osteogenic protein in the composition. Generally, the higher the activity of the protein, the greater the content of the mineral matrix required.

In certain embodiments of the invention, the first implant material includes a plurality of discrete mineral particle such as granules or may be provided by a monolithic synthetic ceramic or other mineral body dimensioned to occupy the desired three dimensional space for bone ingrowth. In other embodiments of the invention, the first implant material includes a porous matrix material incorporating mineral particles. The porous matrix material can be collagenous. A wide variety of collagen materials are suitable for these purposes. Naturally occurring collagens may be subclassified into several different types depending on their amino acid sequence, carbohydrate content and presence or absence of disulfide cross-links. Types I and III collagen are two of the most common subtypes of collagen. Type I collagen is present in skin, tendon and bone whereas Type III collagen is found primarily in skin. The collagen in the matrix may be obtained from skin, bone, tendon, or cartilage and purified by methods known in the art. Alternatively, the collagen may be purchased commercially. The porous matrix composition desirably includes Type I bovine collagen.

The collagen of the porous resorbable matrix can further be atelopeptide collagen and/or telopeptide collagen. Moreover, both non-fibrillar and fibrillar collagen may be used. Non-fibrillar collagen is collagen that has been solubilized and has not been reconstituted into its native fibrillar form.

The resorbable matrix of the first implant material may also be formed of other natural or synthetic polymeric materials, in addition to or as an alternative to collagen. For example, the resorbable matrix may comprise gelatin (e.g. foamed gelatin), or resorbable synthetic polymers such as polylactic acid polymers, polyglycolic acid polymers, or co-polymers thereof. Other natural and synthetic polymers are also known for the formation of biocompatible resorbable matrix materials, and can be used in the invention.

In certain forms of the invention, the first implant material will have a particulate mineral:resorbable porous matrix weight ratio of at least about 4:1, more typically at least about 10:1. In highly mineralized implants, the particulate mineral will constitute at least 95% by weight of the first implant material. For example, highly effective first implant materials are provided wherein they comprise about 97% to about 99% by weight particulate mineral and about 1% to about 3% of the collagen or other matrix forming material. Moreover, the mineral component in certain embodiments has an average particle size of at least about 0.5 mm, more preferably about 0.5 mm to about 5 mm, and most preferably about 1 mm to about 3 mm.

To make one form of the first implant material, a collagen slurry may be prepared as known, and can be chilled to increase its viscosity to help suspend the particulate mineral component. The particulate mineral is dispersed into the collagen slurry and gently mixed. After the particulate mineral component is uniformly dispersed in the slurry, the slurry is poured into sterile trays or other forms and freeze dried. The sheets of implant material are then removed from the freeze drier and if desired exposed to a glutaraldehyde or other cross-linking agent. The composite material formed is desirably three-dimensionally stable but flexible, and can be sterilized and packaged in accordance with known procedures.

As noted above, osteogenic implants of the invention include a second resorbable implant material covering at least a portion of the surface of the first resorbable implant material. The second implant material can include a porous matrix prepared with a matrix-forming material such as those discussed above for the first implant material. Accordingly, the second implant material may include a resorbable collagenous matrix in certain embodiments, which may incorporate any of the collagen types discussed above or any combination thereof. In a particular embodiment, the second implant material may be provided by an absorbable collagen sponge (ACS) material made with Type 1 bovine collagen and manufactured by Integra Lifesciences. As well, the resorbable matrix in the second implant material may also be formed of other natural or synthetic polymeric materials in addition to or as an alternative to collagen. Such materials may for example include gelatin or other natural or synthetic polymers (e.g. polylactic acid, polyglycolic acid, or copolymers thereof) useful for the formation of biocompatible resorbable matrix materials. The second resorbable implant material may also include a mineral component, which may be the same as or different from that of the first resorbable implant material.

As indicated above, osteogenic implants of the invention include an osteogenic protein carried in the second implant material; for example, the osteogenic protein can be a bone morphogenic protein (BMP). Recombinant human BMPs can be used, and may be commercially obtained or prepared as described and known in the art, e.g. in U.S. Pat. No. 5,187,076 to Wozney et al.; U.S. Pat. No. 5,366,875 to Wozney et al.; U.S. Pat. No. 4,877,864 to Wang et al.; U.S. Pat. No. 5,108,932 to Wang et al.; U.S. Pat. No. 5,116,738 to Wang et al.; U.S. Pat. No. 5,013,649 to Wang et al.; U.S. Pat. No. 5,106,748 to Wozney et al; and PCT Patent Nos. WO93/00432 to Wozney et al.; WO94/2693 to Celeste et al.; and WO94/26892 to Celeste et al. The osteogenic protein may be isolated from tissue sources such as bone. Methods for isolating BMP from bone are described, for example, in U.S. Pat. No. 4,294,753 to Urist and Urist et al., PNAS 371, 1984.

In some embodiments, the osteogenic protein will include a pair of polypeptides having amino acid sequences each comprising a sequence that shares a defined relationship with an amino acid sequence of a reference morphogenic protein. Desirable osteogenic polypeptides for use in the present invention have an amino acid sequence that shares a defined relationship with a sequence present in osteogenically active human BMP-2 (SEQ ID NO: 2; see also National Center for Biotechnological Information (NCBI) Accession No. P12643), osteogenically active human BMP-4 (SEQ ID NO: 4; see also NCBI Accession Nos. P12644, and BAA06410), osteogenically active human BMP-6 (SEQ ID NO: 6; see also NCBI Accession No. P22004), or osteogenically active human BMP-7 (SEQ ID NO: 8; see also NCBI Accession No. P18075). However, any one or more of the naturally occurring or biosynthetic sequences disclosed herein similarly could be used as a reference sequence. Polypeptides in a dimeric protein with osteogenic activity can each comprise a sequence that corresponds to a reference sequence or that is functionally equivalent thereto.

Functionally equivalent sequences include functionally equivalent arrangements of cysteine residues disposed within the reference sequence, including amino acid insertions or deletions which alter the linear arrangement of these cysteines, but do not materially impair their relationship in the folded structure of the dimeric morphogen protein, including their ability to form such intra- or inter-chain disulfide bonds as may be necessary for morphogenic activity. Functionally equivalent sequences further include those wherein one or more amino acid residues differs from the corresponding residue of a reference sequence, e.g., the C-terminal cysteine domain (also referred to herein as the conserved cysteine skeleton) of human BMP-2, provided that this difference does not destroy bone morphogenic activity. Conservative substitutions of corresponding amino acids in the reference sequence may be used. Amino acid residues that are conservative substitutions for corresponding residues in a reference sequence are those that are physically or functionally similar to the corresponding reference residues, e.g., that have similar size, shape, electric charge, chemical properties including the ability to form covalent or hydrogen bonds, or the like. Common conservative substitutions are those fulfilling the criteria defined for an accepted point mutation in Dayhoff et al. (1978), 5 Atlas of Protein Sequence and Structure, Suppl. 3, ch. 22 (pp. 354-352), Natl. Biomed. Res. Found., Washington, D.C. 20007.

Conservative substitutions typically include the substitution of one amino acid for another with similar characteristics, e.g., substitutions within the following groups: valine, glycine; glycine, alanine; valine, isoleucine, leucine; aspartic acid, glutamic acid; asparagine, glutamine; serine, threonine; lysine, arginine; and phenylalanine, tyrosine. The term “conservative variation” also includes the use of a substituted amino acid in place of an unsubstituted parent amino acid provided that antibodies raised to the substituted polypeptide also immunoreact with the unsubstituted polypeptide.

As described above, particularly useful sequences for the present invention include those comprising the sequences for BMP-2 or BMP-4 (see WO88/00205, U.S. Pat. No. 5,013,649 and WO91/18098), BMP6 (see WO90/11366, PCT/US90/01630), and BMP-7 (also referred to as OP1, see U.S. Pat. No. 5,011,691 and Oppermann et al.), and functionally equivalent sequences thereto. Publications disclosing these sequences, as well as their chemical and physical properties, include: BMP-2 and BMP-4: WO88/00205, Wozney et al. (1988) Science 242:1528-1534); BMP-7 (OP-1): U.S. Pat. No. 5,011,691, U.S. Pat. No. 5,266,683, Ozkaynak et al. (1990) EMBO J. 9: 2085-2093; and BMP-6: Celeste et al. (1991) PNAS 87: 9843-9847. Recombinant human BMP-2 (rhBMP-2), recombinant human BMP-4 (rhBMP-4), recombinant human BMP-6, recombinant human BMP-7 (rhBMP-7) or heterodimers thereof, may be used to particular advantage. It will be understood, however, that other BMP proteins may be used in the present invention, including for example BMP-9.

In other embodiments, useful proteins include biologically active biosynthetic constructs, including novel biosynthetic morphogenic proteins and chimeric proteins designed using sequences from two or more known morphogens.

In certain embodiments, bone morphogenic proteins useful in aspects of the invention include those in which the amino acid sequences comprise a sequence sharing at least 70% amino acid sequence homology or “similarity”, and preferably 80% homology or similarity, with a reference morphogenic protein selected from the foregoing naturally occurring proteins. Preferably, the reference protein is human BMP-2, human BMP-4, human BMP-6, or human BMP-7, and the reference sequence thereof is the C-terminal cysteine domain present in osteogenically active forms of these proteins. A polypeptide suspected of being functionally equivalent to a reference morphogen polypeptide can be aligned therewith using the method of Needleman, et al. (1970) J. Mol. Biol. 48:443-453, implemented conveniently by computer programs such as the Align program (DNAstar, Inc.). Internal gaps and amino acid insertions in the candidate sequence are ignored for purposes of calculating the defined relationship, conventionally expressed as a level of amino acid sequence homology or identity, between the candidate and reference sequences. “Amino acid sequence homology” is understood herein to include both amino acid sequence identity and similarity. Homologous sequences share identical and/or similar amino acid residues, where similar residues are conservative substitutions for, or “allowed point mutations” of, corresponding amino acid residues in an aligned reference sequence. Thus, a candidate polypeptide sequence that shares 70% amino acid homology with a reference sequence is one in which any 70% of the aligned residues are either identical to, or are conservative substitutions of, the corresponding residues in a reference sequence. In a currently preferred embodiment, the reference sequence is BMP-2. Bone morphogenic proteins useful herein accordingly include allelic, phylogenetic counterpart and other variants of the preferred reference sequence, whether naturally-occurring or biosynthetically produced (e.g., including “muteins” or “mutant proteins”), as well as novel members of the general morphogenic family of proteins, including those set forth and identified above. Certain particularly preferred morphogenic polypeptides share at least 60% amino acid identity with the preferred reference sequence of human BMP-2, still more preferably at least 80% amino acid identity therewith.

In still other embodiments, useful osteogenically active proteins have polypeptide chains with amino acid sequences comprising a sequence encoded by a nucleic acid that hybridizes, under any or all of low, medium or high stringency hybridization conditions, to DNA or RNA encoding reference morphogen sequences, e.g., C-terminal sequences defining the conserved seven cysteine domains of BMP-2 (SEQ. ID NO. 1; see also NCBI Accession No. NM001200), BMP-4 (SEQ. ID NO. 3; see also NCBI Accession Nos. NM001202; NM130850; and NM130851), BMP-6 (SEQ. ID NO. 5; see also NCBI Accession No. NM001718) or BMP-7 (SEQ. ID NO. 7; see also NCBI Accession No. NM001719), and the like. As used herein, high stringent hybridization conditions are defined as hybridization according to known techniques in 40% formamide, 5×SSPE, 5×Denhardt's Solution, and 0.1% SDS at 37° C. overnight, and washing in 0.1×SSPE, 0.1% SDS at 50° C. Standard stringency conditions are well characterized in commercially available, standard molecular cloning texts. See, for example, Molecular Cloning A Laboratory Manual, 2nd Ed., ed. by Sambrook, Fritsch and Maniatis (Cold Spring Harbor Laboratory Press: 1989); DNA Cloning, Volumes I and II (D. N. Glover ed., 1985); Oligonucleotide Synthesis (N.J. Gait ed., 1984): Nucleic Acid Hybridization (B. D. Hames & S. J. Higgins eds. 1984); and B. Perbal, A Practical Guide To Molecular Cloning (1984).

Proteins useful in the present invention generally are dimeric proteins comprising a folded pair of polypeptides. Such morphogenic proteins are inactive when reduced, but are active as oxidized homodimers and when oxidized in combination with others of this invention to produce heterodimers. Thus, members of a folded pair of morphogenic polypeptides in a morphogenically active protein can be selected independently from any of the specific polypeptides mentioned above.

Bone morphogenic proteins useful in the invention include proteins comprising any of the polypeptide chains described above, whether isolated from naturally-occurring sources, or produced by recombinant DNA or other synthetic techniques, and includes allelic and phylogenetic counterpart variants of these proteins, as well as muteins thereof, and various truncated and fusion constructs. Deletion or addition mutants also are envisioned to be active, including those that may alter the conserved C-terminal cysteine domain, provided that the alteration does not functionally disrupt the relationship of these cysteines in the folded structure. Accordingly, such active forms are considered the equivalent of the specifically described constructs disclosed herein. The proteins may include forms having varying glycosylation patterns, varying N-termini, a family of related proteins having regions of amino acid sequence homology, and active truncated or mutated forms of native or biosynthetic proteins, produced by expression of recombinant DNA in host cells.

The bone morphogenic proteins contemplated herein can be expressed from intact or truncated cDNA or from synthetic DNAs in prokaryotic or eukaryotic host cells, and purified, cleaved, refolded, and dimerized to form morphogenically active compositions. Candidate host cells include, without limitation, prokaryotes including E. coli, or eukaryotes including yeast, or mammalian cells, such as CHO, COS or BSC cells. One of ordinary skill in the art will appreciate that other host cells can be used to advantage. Detailed descriptions of specific bone morphogenic proteins useful in the practice of this invention, including how to make, use and test them for osteogenic activity, are disclosed in numerous publications, including for example those referenced hereinabove. Additional osteogenic proteins that may be used in aspects of the present invention are included in the group of osteogenic proteins identified in U.S. patent application Ser. No. 09/045,331 filed Mar. 20, 1998, published Aug. 23, 2001 as US 20010016646 A1.

Other therapeutic growth factors may also be used in accordance with the present invention, especially those that may be used to stimulate bone formation. Such proteins are known and include, for example, platelet-derived growth factors, insulin-like growth factors, cartilage-derived morphogenic proteins, growth differentiation factors such as growth differentiation factor 5 (GDF-5), and transforming growth factors, including TGF-α and TGF-β.

Thus, in view of this disclosure and the knowledge available in the art, skilled genetic engineers can isolate genes from cDNA or genomic libraries of various different biological species, which encode appropriate amino acid sequences, or construct DNAs from oligonucleotides, and then can express them in various types of host cells, including both prokaryotes and eukaryotes, to produce large quantities of active proteins capable of stimulating endochondral bone morphogenesis in a mammal.

In one mode of preparing osteogenic implants of the invention, the second implant material can be positioned over a surface of a body of the first implant material, either before or after the osteogenic protein has been incorporated into the second implant material. For example, a sheet of the second implant material having the osteogenic protein incorporated therein can be wrapped around a block or other substantial three dimensional volume of the first implant material, to prepare an osteogenic implant of the invention. In certain modes of practicing the invention, the osteogenic implant including the combination of the first and second resorbable implant materials can be sized for receipt at a location between two adjacent vertebrae of a mammal, including a human, and can be configured to facilitate fusion of the two vertebrae. In one specific embodiment, the osteogenic implant is configured for insertion between adjacent transverse processes of a human patient, e.g. in the lumbar spine, so as to occupy the spatial volume therebetween with the second material incorporating the osteogenic protein in contact with the transverse processes. Implants so configured can be effectively used to achieve posterolateral fusion in human or other patients in need thereof, including lumbar posterolateral fusion. Such posterolateral fusion procedures can be performed as open surgical procedures or minimally invasive procedures, and can be instrumented or non-instrumented. Minimally invasive procedures can be facilitated by specialized systems, such as the CD Horizon® Sextant percutaneous rod insertion system available from Medtronic Sofamor Danek.

Implants of the invention can also be used in other spinal fusion procedures including anterior and posterior lumbar spinal fusion procedures. For example, implants of the invention can be used on the lamina in posterior fusion or within the disc space, e.g. in interbody fusion techniques. Relatedly, implants of the invention can be used in conjunction with load bearing spinal implants such as fusion cages, and may serve to induce bone growth in, through and/or around such load bearing spinal implants.

Still further, implants of the invention can be used to promote bone growth from and between bone surfaces in other areas of the body, including for example in the repair of long bone defects or cranial defects, including but not limited to the repair of simple and compound fractures and non-unions.

With reference now to FIGS. 1-3, an illustrative osteogenic implant 11 of the invention will be described. Osteogenic implant 11 includes a first implant material 12 forming an implant body for occupying a substantial three-dimensional volume through which bone growth is desired. First implant material 12 can, for example, have a height h¹ of about 1 cm to about 10 cm, a width w¹ of about 0.5 cm to about 2 cm, and a thickness t¹ of about 0.5 cm to about 1.5 cm. Implant 11 also includes a second implant material 13 generally in sheet form, that is used to cover at least a portion of the outer surface of implant material 12 and can in certain embodiments completely encase and cover all surfaces of implant material 12. As illustrated in FIG. 1, sheet form implant material 13 can be folded around implant material 12 (see arrows) to form an osteogenic implant of the present invention including combined first and second matrix materials. Implant material 13 can have suitable dimensions for this purpose, for example having a height h² of about 1 cm to about 10 cm, a width w² of about 1 cm to about 10 cm, and a thickness t² of about 0.2 cm to about 0.5 cm. Unless stated otherwise, the dimensions given herein for the first implant material 12 and the second implant material 13 are their dimensions when wet (saturated).

With particular reference now to FIG. 2, shown is implant 11 of the invention including first implant material 12 encased by implant material 13. Implant material 12 in the illustrated embodiment includes a matrix forming material 14 and mineral particles 15 embedded therein. Second implant material 13 is shown wrapped around first implant material 12 forming an interface I¹ therebetween. In the illustrated embodiment, second implant material 13 wraps completely around first implant material 12 and contacts itself at interface I². In accordance with aspects of the present invention, second implant material 13 will be impregnated with a liquid carrier including an osteogenic protein such as a BMP, and first implant material 12, at least as the implant 11 is assembled, will be free or substantially free from the osetogenic protein incorporated in second implant material 13. It is expected in this regard that once implant 11 is assembled, some level of diffusion of osteogenic protein across interface I¹ may occur; however, in certain embodiments of the invention, it is nonetheless expected that at least a substantial internal volume of the implant body formed from first implant material 12 will remain essentially free from any such osteogenic protein and will be osteoconductive and not osteoinductive in nature. As well, in the illustrated embodiment, first implant material 12 contains mineral particles 15, whereas second implant material 13 is free of mineral particles, while the matrix forming material of implant materials 12 and 13 may be the same, e.g., collagen. In such an arrangement, the first implant material 12 can be more resistant to compressive forces than second implant material 13, with both structures maintaining three-dimensional stability and a flexible or pliable nature. Such implants are advantageously facile in use, and in effectively utilizing osteogenic proteins such as BMPs dosed to the patient with implant 11.

Referring now to FIG. 3, shown are two osteogenic implants 11 of the invention in a schematic representation wherein they can facilitate posterolateral fusion in a human patient. A bilateral fusion is shown, between a first vertebra V¹ and a second vertebra V². In such a procedure, a first osteogenic implant 11 traverses the space between a transverse process TP^(1a) of V¹ and transverse process TP^(2a) of V². A similar arrangement is shown on the opposite side wherein an implant 11 contacts transverse process TP^(1b) of the V¹ and transverse process TP^(2b) of vertebrae V², and traverses the space therebetween. Osteogenic implants 11 thereby induce bone growth from the surfaces of the contacted transverse processes, which bone growth effectively extends through the volume occupied by the osteogenic implants 11, resulting in arthrodesis of the transverse processes and fusion of vertebrae V¹ with vertebrae V². If desired, for such procedures the surfaces of the involved transverse processes may be decorticated to facilitate the fusion process. Techniques and implements for decortication are well known to those of ordinary skill in the art and can be used within the scope of the invention.

The dimensions of implant bodies formed from the first implant material may vary depending on the application. For posterolateral fusion devices for humans, these dimensions may for example be about 3 cm to about 6 cm in height (h¹), about lcm to about 2 cm in width (w¹), and about 0.5 to about 1.5 cm in thickness (t¹). The dimensions for the second implant material may likewise vary. Illustrative implant devices for posterolateral fusion for humans can include an implant body of the first implant material sized as noted above, combined with a second implant material having a height (h²) of about 3 cm to about 6 cm, a width (w²) of about 3 to about 7 cm, and a thickness (t²) of about 0.2 cm to about 0.5 cm. The total volume of implant material (first plus second implant material) for human posterolateral fusion implants will be sufficient to provide the desired fusion mass (e.g. including one level or two level fusions), and may for example range from about 5 cubic centimeters (cc's) to about 20 cc's when the implant materials are wet (saturated).

In this same vein, the total dose of osteogenic protein included in an osteogenic implant of the invention will be sufficient to induce the desired bone growth through the volume occupied by the implant. In a posterolateral fusion implant, the total dose of osteogenic protein will be sufficient to induce the desired intertransverse process fusion mass in combination with the implant, and in the case of a bone morphogenic protein such as BMP-2 (including recombinant human BMP-2, rhBMP-2) this total dose may for example not exceed about 12 mg, e.g. typically range from about 1 mg to about 12 mg, more typically about 3 mg to about 9 mg, including in human fusions. As noted above, this dosed amount of osteogenic protein may be distributed regionally within the implant material. Thus, all or substantially all of this dosed protein may be carried by the second implant material; or, in certain other embodiments, the second implant material may carry a concentration (mg per cc of implant material) that is higher than the concentration carried by the first implant material. In any case, the osteogenic protein may be substantially homogeneously distributed through the first and/or second implant material, or may be regionally concentrated within the implant material, e.g. as a coating.

It will be understood that implants of the invention may also include more than one piece of the first or second implant material. For example, multiple pieces of the first implant material may be wrapped within a single piece of the second implant material, or multiple pieces of the second implant material may cover various portions of the surface of a monolithic implant body formed from the first implant material. Illustratively, separate pieces of the second implant material carrying the osteogenic protein may be positioned overtop a monolithic body of the first implant material and positioned to contact bone surfaces to be fused.

Another embodiment of the invention, in which the first implant material is provided as multiple pieces rather than a single piece, is shown in FIG. 4. In particular, implant construct 20 includes a first (inner) implant material constituted by a plurality of discrete ceramic granules 21, for example synthetic biphasic calcium phosphate granules 21. These granules are present in sufficient quantity to occupy a desired three-dimensional volume into which bone growth is desired, for example a volume into which a spinal fusion mass such as an interbody fusion mass or a posterolateral fusion mass. Granules 21 provide the first implant material, which is wrapped within a second (outer) implant material 22 which is preferably provided by a porous sponge matrix, especially a porous collagen sponge matrix. The second implant material carries an osteogenic protein such as a BMP. In certain embodiments, an aqueous formulation including the BMP or other osteogenic protein is used to wet the first implant material, such that the formulation is sorbed into the implant material. The implant construct 20 can thereafter be implanted into a patient so that the second implant material 22 carrying the ostegenic protein contacts bone surfaces to be fused together, whereupon the growth of a fusion bone mass is induced. In one embodiment, implant construct 20 can be configured for posterolateral spinal fusion, having a length sufficient to traverse the spinous processes to be fused.

One particularly advantageous implant that can be used for human spinal fusion, including posterolateral fusion, has a BMP such as rhBMP-2 in a liquid solution dispersed substantially homogeneously through the second implant material to provide a concentration of at least about 0.4 mg/cc based upon the wet (saturated) volume of the implant, more preferably at least about 0.6 mg/cc, e.g. in the range of about 0.6 mg/cc to about 4 mg/cc, with the first implant material being free from the osteogenic protein other than that which might diffuse from the second material into the first material upon contact between the two materials, e.g. during preparation, implantation or residence in the patient. Such implants can be prepared, for instance, by infusing the second implant material with a solution of the osteogenic protein and thereafter positioning it around the first implant material.

In certain embodiments, measures are taken to localize the majority of the osteogenic protein to the second (outer) implant material and minimize substantial diffusion therefrom that might deleteriously reduce the concentration of osteogenic protein (in mg per cc of wet implant material) in the outer regions of the overall implant that contact and initiate bone growth from adjacent bone surfaces. For example, the second implant material can be effective to bind the BMP or other osteogenic protein by ionic and/or hydrogen bonding or other bonding forces. For example, collagenous sponge material exhibits the capacity to bind BMPs such as BMP-2 in a non-covalent fashion, and the amount of protein that is effectively retained by the material against fusion or wash-out increases over the time of contact with an aqueous BMP formulation. Thus, embodiments of the invention are provided wherein an aqueous formulation of the BMP or other osteogenic protein is applied to the second implant material and allowed to equilibrate for a period of time prior to contact with the first implant material. This equilibration period can last for at least about 2 minutes, at least about 5 minutes, and is typically in the range of about 5 to about 30 minutes.

The equilibration or other protein retaining technique will desirably maintain at least about 70% by weight of the BMP or other osteogenic protein localized to the second implant material at least during the manipulation and implantation of the combined implant construct into the patient. In certain embodiments, at least about 80% by weight of the osteogenic protein will be so retained, even at least about 90% or more. It will thus be understood that certain amounts of the osteogenic protein may migrate from the second implant material in these embodiments, including for example some level of diffusion into the first implant material; however, in these embodiments any amount of osteogenic protein in the first implant material due to diffusion or otherwise will be relatively low compared to that in the second implant material, and the osteogenic protein applied to the second implant material will be substantially retained therein at least to the point of implant as discussed above. In this manner, an effective, high spatial concentration of the BMP or other osteogenic protein can be maintained at the outer regions thereof sufficient to stimulate bone ingrowth from bone surfaces that are contacted.

The present invention also includes kits for promoting bone growth in patients, wherein the kits include a first implant material as described herein, a second implant material as described herein, and an osteogenic protein as described herein. Each of such components of the kit may be provided for example in a lyophilized or otherwise dry state, or in a wet state. The kits can include a structural element in which the components are stably held spaced from one another, in a sterile, medically acceptable packaging system. Such kits can likewise include instructions for use of the kit components for promoting bone growth within a patient, for example a spinal fusion procedure such as a posterolateral spinal fusion procedure. Kits of the invention can thus also include other components such as syringes, vials, surgical instruments for minimally invasive or open techniques, spinal rods, spinal cages or other load-bearing interbody fusion devices, spinal plates, bone screws, and the like.

The invention will now be described with reference to certain specific Examples. It will be understood that these Examples are illustrative and not limiting of the invention.

EXAMPLE 1 Preparation of Collagen Sponge/Bone Particle Composite

12 grams of biphasic calcium phosphate particles (containing 85% tricalcium phosphate and 15% hydroxyapatite), 1 mm in diameter, are added to 12 grams of collagen slurry (0.192 grams of collagen). This composite slurry is poured into a 7.5 cm×10.0 cm mold, freeze dried, double sterile packaged, and sterilized by ETO gas sterilization.

EXAMPLE 2 Posterolateral Fusions Using Combined Matrix Implants

2.1 Materials and Methods

The entire protocol for this Example was reviewed and approved by the Institutional Animal Care and Use Committee for Emory University.

2.1.1 Surgical Procedure

Nine skeletally mature rhesus macaques underwent single level posterolateral intertransverse process spinal arthrodesis under general anesthesia. Anesthesia was induced with 3-5 mg/kg of intramuscular or subcutaneous telazol, and maintained with 1%-2% inhalational halothane. The monkeys were placed prone on the operating table with chest supports, then shaved, prepped and draped in a sterile manner for lumbar surgery.

A manual palpation of the iliac crests was used to estimate the L4-5 vertebral level using a preoperative lateral plain film. Subsequently, 10 mL of bupivicane was used to infiltrate the lumbodorsal region, and a midline incision was made to expose the lumbodorsal fascia. Bilateral fascial incisions were made approximately 2-3 cm off the midline, and a Wiltse muscle-splitting technique was used to develop the plane between the multifidous and logissiumus muscles. The transverse processes of L4 and L5 and the intertransverse membrane were exposed, while leaving the facet joints intact. The dorsal aspects of the L4 and L5 transverse processes were decorticated using a high-speed burr, until bleeding surfaces with cancellous bone were noted. Graft materials (see below) were then placed in the paraspinal muscle bed between the transverse processes. Absorbable 3-0 sutures were used to close the fascia, and the skin was closed using both staples and 3-0 absorbable sutures.

Animals received 0.1 mg/kg bupinorphine when indicated for postoperative pain control and were individually housed. There was no postoperative restriction on activity and no supportive orthotic devices were used. The monkeys were fed a regular diet on a routine basis for the animal facility.

Recombinant human bone morphogenic protein-2 (rhBMP-2) (Medtronic Sofamor Danek, Memphis Term.) was delivered from a stock concentration of 1.5 or 3.0 mg/mL. The compression resistant matrix (CRM) (Medtronic Sofamor Danek, Memphis Term.) was comprised of a bovine type I collagen sponge impregnated with 15% hydroxyapatite/85% tricalcium phosphate ceramic granules, and was prepared generally as described in Example 1. The CRM block was 3.5 cm in height, and 1.2 cm in both width and thickness. The total volume of the CRM implant was about 5.0 cc. The dimensions of the dry absorbable collagen sponge (Medtronic Sofamor Danek, Memphis, Tenn.) were 5×3.8×0.35 cm.

The animals were divided into three groups (n=3 for each group) and had one of the following graft configurations implanted bilaterally as described above: 1) rhBMP-2 (10 mg per side) delivered directly on the CRM carrier; 2) rhBMP-2 (3 mg/side) delivered directly on the CRM carrier; and 3) rhBMP-2 (3 mg/side) delivered on the absorbable collagen sponge, allowed to equilibrate for about 15 minutes, and then wrapped around a block of CRM carrier.

2.1.2 Assessment of Spine Fusion

All animals were euthanized at 24 weeks postoperatively with intravenous pentobarbital. Subsequently, the lumbar spines were removed, and arthrodesis was assessed blindly by 4 methods: 1) manual palpation, 2) posteroanterior plain radiographs, 3) computerized tomography (CT), and 4) undecalcified histology.

After harvesting, the lumbar spines were manually palpated at the level of attempted fusion by a blinded observer. The observer also palpated the superior and inferior adjacent motion segments. Each motion segment was considered fused only if there was no motion present, otherwise it was graded as not fused.

Radiographs of each spine were made using a tube to plate distance of 90 cm. The radiographs were then reviewed in a blinded method; only those radiographs showing a continuous pattern of trabecular bone in the intertransverse fusion mass were graded as fused.

All lumbar spine specimens underwent CT scans in the region of the arthrodesis. A high speed spiral CT scanner (GE, Milwaukee, Wis.) was utilized, using the following parameters: 100 cm field of view, 150 mA, 100 kV, 1 mm gap, and 1 mm slice thickness. The continuity of the fusion mass and any bone formation outside the fusion mass were evaluated.

Histologic analysis was performed after the lumbosacral spines were fixed for 24 hours in a 10% neutral-buffered solution. The specimens were then placed in 70% ethanol, trimmed, and sequentially dehydrated in 95% and 100% ethanol. This step was followed by a xylene treatment. The specimens were then divided in half in the midsagittal plane, embedded in methylmethacrylate, and sectioned to 25 micrometer thickness in a sagittal or coronal plane using an automated system Exakt Technologies, Inc., Oklahoma City, Okla.). The sections were stained with 1% methylene blue and 0.3% basic fuchsin. The sections were then evaluated for the presence of newly formed trabecular bone. Histologic fusion was considered to be present if there was continuous new bridging bone across the carrier connecting the two transverse processes.

2.2 Results

2.2.1 Manual Palpation

All animals survived surgery and had uneventful postoperative courses. The three monkeys that received 10 mg of rhBMP-2 on the CRM carrier achieved solid fusions. The three monkeys that had 3 mg rhBMP-2 implanted on the CRM carrier did not achieve solid fusions. The three monkeys that received 3 mg rhBMP-2 on the absorbable collagen sponge which was then wrapped around the CRM carrier achieved solid fusions.

2.2.2 Radiographs

Serial plain radiographs were taken at 4-6 week intervals and confirmed the manual palpation results. Early fusion masses were visible on the plain films by 12 weeks and by 8 weeks on CT scans in most cases. The serial CT scan results also paralleled the plain radiographs. It was much easier to interpret the presence and extent of new bone formation in the posterolateral spine using CT scans. The three monkeys that did not achieve solid fusions with the 3 mg rhBMP-2 placed directly on the CRM carrier showed some spotty bone formation, especially around the decorticated transverse processes, but it was minimal and not nearly enough to form a continuous bridge of bone.

2.2.3 Histology

Histologic analysis of the fusion masses demonstrated consistent findings with the CT scans. Normal appearing mature trabecular bone was present with marrow cavities in the six monkeys with solid spine fusions. There was no evidence of abnormal inflammatory cells or other reaction to the carrier. In the six animals with the solid fusions, the CRM carrier had been completely remodeled. In the three animals with the nonunions, most of the CRM carrier had been resorbed.

EXAMPLE 3 Medical Kit and Preparation of Combined Implant Material Constructs

A medical kit is provided including a vial containing sterile lyophilized rhBMP-2 (12 mg); a collagen sponge (Absorbable Collagen Sponge (ACS), Integra Lifesciences) 3″×4″ in size (7.5 cm×10 cm) packaged in a tray; a vial containing sterile water for injection (10 ml); two 10 ml syringes; two 20 G 1½″ needles; and instructions as to the following preparation.

Using one of the needles and a 10 ml syringe, the rhBMP-2 is reconstituted with 8.4 ml of sterile water for injection in a vial. The rhBMP-2 is gently swirled in the vial during reconstitution. The ACS is cut in half making two pieces each dimensioned 2″×3″. The ACS is placed in the packaging tray. Using a second needle and 10 ml syringe, 4 ml of rhBMP-2 are withdrawn from the vial. 4 ml of rhBMP-2 solution is distributed onto one 2″×3″ piece of ACS. The second needle/syringe is used to withdraw another 4 ml of the rhBMP-2 solution from the vial, which is distributed uniformly onto the second piece of ACS. The ACS pieces are allowed to stand for a minimum of 15 minutes (and should be used for implantation within the next 60 minutes). A 10 cc vial of MasterGraft™ granules (10 cc, biphasic calcium phosphate having an 85:15 tricalcium phosphate:hydroxyapatite ratio) is divided into two equal 5 cc portions. The 5 cc granule portions are each distributed onto one of the 2″×3″ wetted ACS pieces. Using forceps, the rhBMP-2 soaked ACS with MasterGraft™ granules are each rolled into a 2″ wide roll with the ACS surrounding the MasterGraft granules. The combined implant constructs so prepared can be used in a spinal interbody fusion procedure, potentially in combination with metal cages into which the constructs are inserted, or in a posterolateral spinal fusion procedure.

While the invention has been illustrated and described in detail in the foregoing description, the same is to be considered as illustrative and not restrictive in character, it being understood that only certain embodiments have been shown and described and that all changes and modifications that come within the spirit of the invention are desired to be protected. In addition, all publications cited herein are hereby incorporated by reference in their entirety as if each had been individually incorporated by reference and fully set forth. 

1. An osteogenic implant for promoting bone growth between first and second bone surfaces, comprising: a first resorbable implant material defining an implant body configured for receipt between the first and second bone surfaces, said first resorbable implant material comprising a porous collagenous matrix containing mineral particles, said implant body having an outer surface, the first resorbable implant material being substantially free of an osteogenic protein; a second resorbable implant material wrapped around at least a portion of said outer surface of said implant body and positioned to contact said first and second bone surfaces, said second resorbable implant material comprising a porous collagen sheet and being substantially free from mineral particles; and an osteogenic protein carried by said second resorbable implant material.
 2. The osteogenic implant of claim 1, wherein: said implant body is dimensioned for receipt between adjacent upper and lower transverse processes in a human spine.
 3. The osteogenic implant of claim 1, wherein: said osteogenic protein is a bone morphogenic protein (BMP).
 4. The osteogenic implant of claim 3, wherein: said BMP is BMP-2 or BMP-7.
 5. The osteogenic implant of claim 4, wherein: said BMP is BMP-2.
 6. The osteogenic implant of claim 5, wherein: said BMP-2 is recombinant human BMP-2.
 7. The osteogenic implant of claim 1, wherein: said implant body is configured for receipt between adjacent transverse processes in a human spine; and said osteogenic protein is a BMP.
 8. The osteogenic implant of claim 7, wherein: said BMP is BMP-2.
 9. The osteogenic implant of claim 8, wherein: said BMP-2 is recombinant human BMP-2.
 10. The osteogenic implant of claim 1, wherein: said mineral particles comprise bone.
 11. The osteogenic implant of claim 1, wherein: said mineral particles comprise synthetic ceramic.
 12. The osteogenic implant of claim 11, wherein: said synthetic ceramic comprises tricalcium phosphate.
 13. The osteogenic implant of claim 12, wherein: said synthetic ceramic comprises biphasic calcium phosphate.
 14. The osteogenic implant of claim 13, wherein: said biphasic calcium phosphate has a weight ratio of tricalcium phosphate to hydroxyapatite of about 50:50 to about 95:5.
 15. The osteogenic implant of claim 14, wherein: said ratio of tricalcium phosphate to hydroxyapatite is about 80:20 to about 90:10.
 16. The osteogenic implant of claim 15, wherein: said ratio of tricalcium phosphate to hydroxyapatite is about 15:85.
 17. The osteogenic implant of claim 3, wherein: said BMP is substantially homogeneously incorporated in said second resorbable implant material.
 18. The osteogenic implant of claim 17, wherein: said BMP is incorporated in said second resorbable implant material in an amount providing a BMP concentration of at least about 0.4 mg per cc of the second resorbable implant material.
 19. The osteogenic implant of claim 18, wherein: said level is in the range of about 0.6 mg per cc to about 4 mg per cc of the second resorbable implant material.
 20. The osteogenic implant of claim 1, wherein: said second resorbable implant material comprises a collagenous sponge sheet.
 21. An implant configured to promote posterolateral spinal fusion between first and second transverse processes in a patient, comprising: a first resorbable implant material defining an implant body configured for receipt between the first and second transverse processes, said first resorbable implant material comprising a porous collagen-containing matrix incorporating mineral particles, said implant body having an outer surface, the first resorbable implant material being free of a bone morphogenic protein; a collagen sponge sheet wrapped around at least a portion of said outer surface of said implant body and positioned to contact the first and second transverse processes, said collagen sponge sheet being substantially free from mineral particles; and a bone morphogenic protein (BMP) incorporated in said collagen sponge sheet.
 22. The implant of claim 21, wherein: said collagen sponge sheet comprises the BMP at a concentration of about 0.6 mg/cc to about 4 mg/cc.
 23. The implant of claim 22, wherein: said BMP is BMP-2.
 24. The implant of claim 23, wherein: said BMP-2 is recombinant human BMP-2.
 25. The implant of claim 22, wherein: said implant has a total dose of said BMP not exceeding about 12 mg.
 26. The implant of claim 25, wherein: said implant has a total dose of BMP of about 1 mg to about 12 mg.
 27. A medical implant for promoting bone growth between first and second bone surfaces, comprising: a first bioresorbable implant material comprising mineral and configured to occupy a volume for bone in growth between the first and second bone surfaces; and a second bioresorbable implant material comprising a porous collagen sponge matrix sheet and wrapped around at least a portion of said first implant material, said second bioresorbable implant material configured to contact the first and second bone surfaces, said porous collagen sponge sheet being substantially free from mineral particles; said medical implant comprising a total amount of osteogenic factor, and wherein all of said total amount of osteogenic factor is localized to said second bioresorbable implant material.
 28. An implant suitable for carrying an osteogenic protein to induce bone growth between first and second bone surfaces in a patient, the implant comprising: a first resorbable implant material defining an implant body configured for receipt between the first and second bone surfaces, said first resorbable implant material comprising a porous collagenous matrix containing mineral particles, said implant body having an outer surface, the first resorbable implant material being free of an osteogenic protein; and a second resorbable implant material effective to carry an osteogenic protein and to wrap around at least a portion of said outer surface of said implant body and be positioned to contact the first and second bone surfaces, wherein the second resorbable implant material comprises a porous collagen matrix sheet that is substantially free from mineral particles and contains the osteogenic protein.
 29. The implant of claim 28, which is configured for receipt between and contacting adjacent upper and lower transverse processes of a human spine.
 30. A medical kit for promoting bone growth between first and second bone surfaces of a patient, comprising: a first resorbable implant material comprising a matrix having particulate mineral embedded therein, the first resorbable implant material being substantially free of an osteogenic protein; a second resorbable implant material configured to wrap around at least a portion of a surface of the first resorbable implant material, said second resorbable implant material comprising a porous collagen sheet that is substantially free from mineral particles; and an osteogenic protein contained in the collagen sheet.
 31. The medical kit of claim 30, wherein: said first and second resorbable implant materials are dimensioned to form an implant configured for receipt between and contacting adjacent upper and lower transverse processes of a human spine.
 32. A medical implant for promoting bone growth, comprising: an outer osteogenic implant material, the outer osteogenic implant material including a wetted, porous bioresorbable sponge sheet having absorbed therein an aqueous medium including an osteogenic protein, said outer osteogenic implant material not containing an osteoconductive mineral component; and an inner scaffolding implant material including an osteoconductive mineral component, the inner scaffolding implant material being substantially free of the osteogenic protein and configured to occupy a three-dimensional volume for bone ingrowth initiated by said outer osteogenic implant material, wherein said outer osteogenic implant material is wrapped around said inner scaffolding implant material.
 33. The medical implant of claim 32, wherein said medical implant includes a total dose of said osteogenic protein, and wherein all of said total dose is retained by said outer osteogenic implant material.
 34. The medical implant of claim 32, wherein the osteogenic protein is a bone morphogenic protein.
 35. The medical implant of claim 34, wherein the bone morphogenic protein is BMP-2 or BMP-7.
 36. A method for inducing bone growth in a patient, comprising implanting the osteogenic implant of any of claim 1-20, 21-27, 28, or 29 into a patient at a site so as to induce bone growth.
 37. A method for inducing bone growth in a patient, comprising implanting the medical implant of any of claims 32-35 into a patient at a site so as to induce bone growth.
 38. A method for inducing spinal fusion between first and second bone surfaces in a patient, the method comprising implanting the implant of any of claim 2, 7, 21-26, 29, or 31 between adjacent upper and lower transverse processes of a human spine, such that the implant occupies a volume for bone growth to create a fusion mass between the adjacent upper and lower transverse processes of the human spine. 